Shadow chancellor John McDonnell flip-flopped on the fiscal charter on Tuesday (Source: Getty)

Prime Minister David Cameron used today's Prime Minister's Questions (PMQs) to attack the Labour party over its U-turn on the government's so-called fiscal charter.

Cameron confronted Labour leader Jeremy Corbyn at least four times about the party's last-minute switch to opposing the charter, which will require future governments to maintain a surplus – but Corbyn refused to reply directly to Prime Minister's prodding.

Firebrand shadow chancellor John McDonnell sparked outrage among Labour MPs on Monday when he announced that his party would reject the government’s “fiscal charter” less than one month after saying he planned to support it.

McDonnell later defended his last-minute decision by saying that there was a “growing reaction” against the government’s proposed spending cuts and vowing to “underline” Labour’s position as an anti-austerity party by voting against the fiscal charter today.

Earlier today, chancellor George Osborne called on on Labour MPs to defy their party's whip and back the fiscal charter.

"With Labour’s economic policy in obvious chaos, I call on all moderate, progressive Labour MPs to defy their leadership and join with us to vote for economic sanity," Osborne said.

A House of Commons vote on the legislation is scheduled for later today.

In a rare nod to his former counterpart Chris Leslie Osborne added: "Failing that, they should at least follow the advice of the former shadow chancellor and abstain."

“A fortnight ago, Labour told voters they were ready to back our plans. But now, they have confirmed they want to go on borrowing forever – loading debts onto our children that they can never hope to repay," Osborne added.

"This is not socialist compassion – it’s economic cruelty. As Labour’s Great Recession showed, those who suffer most when government run unsustainable deficits are not the richest but the poorest."

Other Labour MPs have taken heed of advice from Osborne and Leslie, with Chris Leslie, MP for Islwyn, saying that Labour should support "a budget surplus in principle" but ensure that certain tests are passed, including ensuring the safety of the NHS, vital public services and national security.

Leslie added: "I have decided to abstain on today's vote on the fiscal charter. I cannot in good conscience support a position which I do not agree with, which has been inconsistent with existing Labour policy and which has not been properly explained by the shadow chancellor and Labour leadership."

The charter for budget responsibility, which was first introduced in parliament over the summer, will require the government to balance its books within three years. From then on, all governments will have to run an overall budget surplus unless the country veers from “normal” conditions.